The human immunodeficiency viruses infect CD4+ macrophages and T helper cells. Although HIV-1 entry requires cell surface expression of CD4, to which the viral envelope glycoproteins bind, several studies have suggested that it is not sufficient for fusion of the viral envelope to the cellular plasma membrane. Early studies have shown that while human cells expressing a transfected CD4 gene were permissive for virus entry, murine cells expressing human CD4 were not. These findings led to the suggestion that there is a species-specific cell surface cofactor required in addition to CD4 for HIV-1 entry. Subsequent studies have shown that strains of HIV-1 that had been adapted for growth in transformed T-cell lines (T-tropic strains) could not infect primary monocytes or macrophages; in contrast, primary viral strains were found to infect monocytes and macrophages, but not transformed T cell lines. This difference in tropism was found to be a consequence of specific sequence differences in the gp120 subunit of the envelope glycoprotein, suggesting that multiple cell type-specific cofactors may be required for entry in addition to CD4.
The nature of the cofactors required for HIV entry proved elusive until the recent identification by Feng et al. of fusin, a member of the seven transmembrane G-protein coupled receptor family. Fusin (CXCR-4) was shown to act as a co-receptor for T-tropic strains; however, it did not support infection of CD4+ cells by macrophage-tropic viruses, which more closely resemble those that predominate in infected individuals throughout the course of the disease, particularly in the asymptomatic phase. In addition, these strains appear to be responsible for HIV-1 transmission, both sexually and by transfer of infected blood. Rare individuals who are resistant to sexual transmission of HIV-1 have T-cells that are readily infected by T-tropic virus, but cannot be infected by macrophage-tropic virus, further supporting a role for macrophage-tropic virus in sexual transmission of HIV-1.
Cocchi et al. recently characterized inhibitors of HIV-1 replication present in supernatants of CD8+ T cells as the β-chemokines RANTES, MIP-1α and MIP-1β. Chemokines are chemotactic cytokines that are released by a wide variety of cells to attract macrophages, T cells, eosinophils, basophils and neutrophils to sites of inflammation (reviewed in ref. 14). The chemokines fall into two classes, C—X—C (α) and C—C (β), depending on whether the first two cysteines are separated by a single amino acid or are adjacent. The α-chemokines such as IL-8, NAP-2 and MGSA are chemotactic primarily for neutrophils, while β-chemokines such as RANTES, MIP-1α, MIP-1β, MCP-1, MCP-2, and MCP-3 are chemotactic for macrophages, T-cells, eosinophils and basophils. The chemokines bind specific cell surface receptors belonging to the family of G protein-coupled seven transmembrane domain proteins (reviewed in Ref. 15). Upon binding their cognate ligands, chemokine receptors transduce an intracellular signal through the associated trimeric G protein. This results in a rapid increase in intracellular calcium concentration. There are at least seven human chemokine receptors that bind or respond to β-chemokines with the following characteristic pattern: CC-CKR1 (MIP-1α, MIP-1β, MCP-3, RANTES), CC-CKR-2A and CC-CKR-2B (MCP-1, MCP-3), CC-CKR-3 (eotaxin, RANTES, MCP-3), CC-CKR-4 (MIP-1α, RANTES, MCP-1), CC-CKR-5 (MIP-1α, RANTES, MIP-1β), and the Duffy blood group antigen (RANTES, MCP-1). These transmembrane receptors could be involved in HIV infection.
Therefore, there is a need to identify a translocation promoting agent that functions in conjunction with CD4 during HIV infection in macrophage. Further, there is a need to provide methods for identifying drugs that can interfere with HIV infection of macrophage by hindering the interaction of CD4, the translocation promoting agent and HIV envelope proteins.